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Nanosized antioxidants are highly advantageous in terms of versatility and pharmacokinetics, with respect to conventional molecular ones. Melanin-like materials, artificial species inspired by natural melanin, combine recognized antioxidant (AOX) activity with a unique versatility of preparation and modification. Due to this versatility and documented biocompatibility, artificial melanin has been incorporated into a variety of nanoparticles (NP) in order to give new platforms for nanomedicine with enhanced AOX activity. In this review article, we first discuss the chemical mechanisms behind the AOX activity of materials in the context of the inhibition of the radical chain reaction responsible for the peroxidation of biomolecules. We also focus briefly on the AOX properties of melanin-like NP, considering the effect of parameters such as size, preparation methods and surface functionalization on them. Then, we consider the most recent and relevant applications of AOX melanin-like NPs that are able to counteract ferroptosis and be involved in the treatment of important diseases that affect, e.g., the cardiovascular and nervous systems, as well as the kidneys, liver and articulations. A specific section will be dedicated to cancer treatment, since the role of melanin in this context is still very debated. Finally, we propose future strategies in AOX development for a better chemical understanding of melanin-like materials. In particular, the composition and structure of these materials are still debated, and they present a high level of variability. Thus, a better understanding of the mechanism behind the interaction of melanin-like nanostructures with different radicals and highly reactive species would be highly advantageous for the design of more effective and specific AOX nano-agents. [ABSTRACT FROM AUTHOR]

This review highlights the progress made in recent years in harnessing the peculiar chemistry of the hydroperoxyl, or perhydroxyl, radical (HOO•) during lipid peroxidation, particularly with regard to its interaction with antioxidants. The HOO• radical, the protonated form of superoxide, plays an important role in the propagation and termination of lipid peroxidation in nonaqueous systems. However, differently from alkylperoxyl (ROO•) radicals that have only oxidizing ability, HOO• has a two-faced oxidizing and reducing activity. The HOO• radical can reduce the radical of the antioxidant (phenols and aromatic amines) by H-atom transfer (A• + HOO• ⟶ AH + O2) thus increasing the length of the inhibition period and the effectiveness of the antioxidant. The simultaneous presence of HOO• and ROO• radicals triggers the catalytic antioxidant activity of quinones and nitroxides and explains the antioxidant activity of melanin-like polymers. The HOO• radical can be formed by fragmentation of ROO• radicals deriving from amines, alcohols, substituted alkenes and may be present at low concentrations in many oxidizing systems. Pro-aromatic compounds, like the natural essential oil component γ-terpinene, are the most effective sources of HOO• and behave as co-antioxidants in the presence of nitroxides or quinones. The future developments and applications of HOO• chemistry in the context of the inhibition of autoxidation are also discussed. [ABSTRACT FROM AUTHOR]

The regeneration of a series of 2,3-dihydrobenzo[b]selenophene-5-ol derivatives by water-soluble N-acetylcysteine in a two-phase model system for lipid peroxidation is investigated. The results have shown that the regeneration and catalytic performance is dictated by the number and position of methyl substituents present in the phenolic ring of the antioxidant.

A kinetic and thermodynamic investigation of phenols para-substituted with thiyl (SR), sulfinyl (SOR) and sulfonyl (S[O.sub.2]R) groups and ortho-substituted with thiyl groups is presented. The ESI-MS analysis of the products that are formed by reacting the sulfides with peroxyl radicals from the azoinitiator AIBM has revealed the formation of a complex mixture of products that might play a vital role in determining the overall antioxidant activity of the parent compounds.

Oxidative stress is a contributing factor in many of the diseases of aging, which include conditions such as atherosclerosis and Alzheimer's disease. Bodies have an endogenic arsenal of enzymatic and nonenzymatic antioxidants to combat oxidative degradation, but as it is apparent that these defenses are sometimes breached, there is an interest in the development and application of new synthetic antioxidants for antioxidant therapy.

The antioxidant activity of the cis and trans isomers of several analogues of resveratrol and pterostilbene is investigated, especially with regard to the effect to the stereochemistry about the olefinic double bond. The antioxidant power of these compounds is estimated by measuring the rate constants for their reactions with peroxyl radicals and, with two of them, the bond dissociation enthalpy of the phenolic O-H bond which is cleaved in the inhibition reaction.

Research has been conducted on alpha-tocopherol. The regeneration of this compound carried out in homogeneous solution by means of coantioxidants in investigating the mechanism responsible for this process is described.

Research has been conducted on the homolytic reactivity of 3,3'-di--tert-butyl-5,5'-dimethyl(1,1'-biphenyl)-2,2'-diol. The thermodynamics and kinetics of this reactivity have been investigated and the results are described.

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Melanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO. (k=1.5×107 and 1.1×105 M−1 s−1 in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones. [ABSTRACT FROM AUTHOR]